Various formation evaluation (FE) tools are used in hydrocarbon exploration and production to measure properties of geologic formations during or shortly after the excavation of a borehole. The properties are measured by formation evaluation tools and other suitable devices, which are typically integrated into a bottom hole assembly (BHA).
FE tools include various types of sensors, such as resistivity sensors, which measure formation resistivity by passing an electrical current through a formation and measuring a change in the current through the formation. This may be accomplished by physically coupling a measurement electrode and a return or ground electrode to the formation, emitting a current into the formation between the measurement and ground electrodes, and measuring electrical potential differences between the electrodes. The resulting voltage/current (V/I) ratios are converted to apparent resistivities, using algorithms based upon the electrode array geometries and potential theory. Alternatively, the electrodes are capacitively coupled to the formation. In other alternatives, a transmitting antenna inductively couples current to the formation, which is in turn received by the same or a separate antenna.
One type of resistivity sensor is a galvanic resistivity sensor, which is galvanically coupled directly to the earth formation with its electrodes. Galvanic resistivity logs are useful, for example, for characterizing stratigraphy, making correlations between wells and inferring formation porosity.
Resistivity sensors produce images of formation resistivity that can be compromised by parasitic factors such as tool standoff, i.e., the distance between the FE tool and a borehole wall, and electronic phase systematic error.